The present invention relates to systems for processing paperboard sheets, such as container blanks, and more particularly to a vacuum assisted sheet transfer device such as is used, for example, to transfer sheets between the printing stations in a sheet-fed printing system.
As is well known in the art, sheet-fed flexographic printing machines are used for multicolor printing on one side of solid or corrugated paperboard blanks which are subsequently converted into containers or cartons. The blanks are fed through multiple rotary print stations and, typically, into a rotary die cutter at the downstream end. The finished blanks are folded and glued, all in a manner well known in the industry. In order to avoid contact with the freshly printed face of the blanks, the blanks are typically transferred between print stations and between the last print station and the rotary die cutter with vacuum-assisted belt conveyor devices that engage the unprinted face of the blank, usually from above, but may be from below. Since accurate registration of the blanks must be maintained from one print station to the next and into the die cutter, vacuum assisted belt transfer devices must be capable of capturing and transferring the blanks reliably and without loss of register. As a result, large volumes of air are required to induce adequate levels of negative pressure in the areas where sheets are present and, of course, correspondingly large blowers must be used. In addition, the large volume airflow is provided continuously and without regard to sheet length or the spacing between sheets. Vacuum-assisted sheet transfer devices are known in which vacuum generated airflow through the sheet transfer system is controlled laterally to accommodate sheets of varying width, but such devices are typically mechanically complex.
Therefore, it would be desirable to have a vacuum distribution system for a belt conveyor sheet transfer device that applies vacuum only when and where a sheet is present, to thereby minimize air volume loss and to permit the use of smaller blowers.